Sigma-delta converters are of great use in modern sophisticated systems using digital signal processing technology. They particularly permit the development of efficient Analog-to-digital (A/D) and Digital-to-Analog (D/A) converters for transforming analog information into a form suitable for digital handling, and vice versa. Numerous architectures for sigma-delta converters exist in the art. An existing, simple, but efficient architecture which is often used is the "double-loop" architecture shown in FIG. 1 and described in detail hereinafter. The prior sigma-delta architecture provides a satisfactory theorical performance and can be embodied in two distinctive ways: a first embodiment using discrete components and a second one using integrated technology.
The double-loop architecture can, firstly, be embodied in discrete components which are wired on a printed circuit board, thus making up a simple and low-cost converter. However, such an embodiment is rarely used in the art in view of the fact that the actual performance which can be obtained with the discrete implementation is far from the expected theorical performance of the double-loop architecture. For instance, for a double-loop sigma-delta architecture leading to a theorical signal-to-noise ratio of about 80 dB, the corresponding converter which uses a discrete implementation will eventually provide a ratio of only 65 dB. The weak performances obtained by the discrete implementation are due to the switching noise coming from the switching components and from the asymmetry of the rise and fall time in the different analog signals and particularly at the output of the digitizer.
In order to improve the actual signal-to-noise ratio of the sigma-delta converter, electronic manufacturers have designed a second embodiment which uses an integrated technology. The actual performance of the integrated sigma-delta converter is far better than that of the previous converter using discrete components because the physical dimensions of the converter are reduced. Also, since the integrated sigma-delta converter consumes less power, the switching noise is reduced.
Thirdly, integrated sigma-delta converters which exist in the art are based on the switched capacitor technology widely used in the field of integrated analog filter design since capacitors are very easy to integrate on a chip. A sigma-delta converter using the switched capacitor technique is described in detail in U.S. Pat. No. 4,746,899 entitled "Method for Reducing Effects of Electrical Noise in an Analog-to-Digital Converter", is further not subject to the above asymmetry problem. As a result, an integrated sigma-delta converter using the switched capacitor technique has good performance and particularly provides an actual signal-to-noise ratio very close to the theorical value, about 78 dB, noted in the above theoretical example. However, the integrated sigma-delta converter presents a major drawback in that it requires the design, development and manufacture of an integrated circuit which is a long and expensive operation.
Therefore a need exists in the art for a simple and low-cost sigma-delta converter made up with common, discrete, (off-the-shelf) components which, nevertheless, provides actual performances in terms of signal-to-noise ratio and linearity which are close to those reached with integrated sigma-delta converters.